Soft elastic materials with a Young's modulus below 1 atm (100 kPa) are vital for the creation of biocompatible implants, substrates, and robots with mechanical properties matching that of live cells, tissues, and organs (Levental, I. et al. (2007) Soft Matter 3: 299-306; Williams, D. F. (2008) Biomaterials 29: 2941-2953; Rus and Tolley (2015) Nature 521: 467-475; Drury and Mooney (2003) Biomaterials 24: 4337-4351). Currently, polymer gels are the only viable class of synthetic materials for low modulus applications, yet with a caveat: their properties are entirely dependent on the fraction of solvent in the system (Langer and Tirrel (2004) Nature 428: 487-492; Wichterle and Lim (1960) Nature 185: 117-118; Pelham and Wang (1997) PNAS 94: 13661-13665; Discher et al. (2005) Science 310: 1139-1143; Anseth et al. (1996) Biomaterials 17: 1647-1657; Baumberger et al. (2006) Nature Mater. 5: 552-555). Solvent is a potential source for various complications including phase separation, drying, and leakage upon deformation that not only compromise the gel elasticity, but may also elicit severe inflammatory response in surrounding tissues (Van Diest et al. (1998) J Clin. Pathol. 51: 493-497).
To lower the modulus of elastomers, one possible approach is to prepare a crosslinked gel from a solution of linear polymers at a low concentration at which they are not entangled, and then graft linear polymers from the network strands between crosslinks. These grafted polymers make the network strand “fat,” effectively increasing its molecular weight, lowering the density of crosslinks, and thus lowering the shear modulus. After removing the solvent molecules, one obtains a “dry,” yet soft network. Despite these advances, the scope of this approach has remained limited due its complex and expensive synthesis, as well as the biocompatibility of the gel due to residual solvent molecules. Accordingly, there remains a need for soft, solvent-free, and biocompatible elastomers.